Radio telephone system



Patented .lune 26, A1928.

unirsi) STATES HAEEY NYQUIST, or MILLBEN, [AND SUMNEE B. WEIGHT, or EAST ORANGE, NEW

JERSEY, AsSieNoas To AMERICAN TELEPHONE AND TELEeEAifHl COMPANY, A oonronATro'N 0E NEW YORK.

'RAnio TELErHoNE sYSTEiii.y

Application iiled` April 23, 1926. 'Serial No. 104,202.

This invention relates to a two-waytele-y phone systemv in which echoes and singingl are suppressed by means of voice. controlled relays, the invention being particularly adapted for use in connection with two-way radio telephone Systems.

In a. radio telephone system in which the same carrier frequency or the same transmission band is used for transmission' in both directions, echo and singing eilects are particularly undesirable because outgoing transmission may be received much stronger at the local receiving station than that des'ired to be received 'from the distant trans mitting station. Another difficulty experienced in systems of this type istliat static disturbances tend to operate the voice conf trolled relays which are used to suppress echoes.

ln order to obviate these diliculties, the. system oi the present invention is so ar-v ranged that ordinary voice operated switching devices are used in the transmitting branches at both ends of the system in order to determine the direction oi" transmission which prevails while a given subscriber is talking. Similar switching devices are also employed at the receiving branches, but in this case they are controlled by means of single frequency currents which are transmitted over the radio channel, these single frequency currents being controlled by relays at the transmitting points. These currents are normally transmitted while no talking is taking` place. scriber talks, the controlling current for that direction is. cut olf, and his voice currents are radiated instead. By using selective circuits whicli are sharply discriminative to receive these single frequency controlling currents. the effect of staticupon the switching devices associated with the receiving channels is relatively small.` Furthermore, the use ot single frequency currents radiated 'from one station to the other for control puposes does not interfere with the full use ot the available power range of the transvniitter for telephone purposes, since the control currents are only radiated at times when the transmitter is not ybeing yused for talking purposes. y

he invention mavnow be more'fully un freni the, following detailed dieL When `the subwhen .readn connect-ien with the v accompanying drawing, Figure 1 of which is a circuit diagram illustrating the preferred embodiment of. the invention; Fig. 2 is a detailed circuit showing a typical delay network; F ig. 3 is `another detail showing a typical selective circuit or voice'pass lter; Fig. 4lsliows the circuit of a typical vacuum tube detector; and Fig. 5 shows lthe detailed circuit arrangements ofthe volumeregulator employed in connectionwith the circuit of Fig. 1. n

. Referring to Fig. 1, a subscribers station S is connected over a transmission line TL to the transmitting branch TE and receiving branch Riv' of a radioy system including transmitter T and receiver R. v,The transmitting and receiving branchesare arranged in the usual manner in the form of va fourwire circuit, the terminals ofl the branches being connected through a hybrid coil to the line TL, which in. turn isbalanced by the usual network N, At the distant end of the system, the subscribers set` S is connected tothe transmission line TL. The transmission line in turn is Lconnected through a hybrid coil and. balancing network N to the'transmitting lbranch'TVV and the receiving branch RE arranged in the form of a iour-wire circuit leading to the vradio trans'n mitter T` and' radio receiver R', respectively. y

`The transmitting .branches TEy and vTW are normally open-circuited at the contactsI ot relays R1 and Rl, respectively. Likewise, thereceiving branches RW and RE are disabled by short circuits controlled by kthe relays R2 and RQ, respectively'. Consequently no free'paths for transmission exist in the circuit in its normal condition.

- Relays `R, and R, are controlled by detectors D1 and-DQ, respectively, which are bridged across the transmitting channels TE and TW. These` detectors may be of any' known type, such, for example, as ya vacuum tube detector ofthe type illustrated in Fig. 4. The detectors `function to rectify orpartially rectify voicecurrents.tansmitted over the transmitting channel to thereby cause the` operation of the relays R1` and RQ. Normally, control currents of diiierent fre-v quencies are sup lied continuously" from the taiismittingstations T and` T byumeaiis Of iierators G and G', respectively "hose circuits are completed over l.a y'

con

iii!

tacts of the relays R, and RQ. These trequeneies are preferably frequencies lying just outside of thev voice band but sutilcicntly separated therefron'i to permit sharp selection. For example, generator G may generate a frequency of cycles Which is impressed upon the radio transmitter' T to modulate the carrier in the usual Way, so that it is radiated and picked up by the distant receiving` set R. Similarly, the generator G may generate a frequency of 3,000 cyclesfivhich is inn'iressed upon the transmitter T to modulate the lcarrier and then be radiated to be picked up by the receiver Tl.

ln connection with the 'receiving channel RlW, a branch circuit is taken ott at the out put side ot the radio receiver R, said branch including a selective circuit F which is sharply selective ot the frequency of 3,000 cycles which will norn'ially be 'detected by the radio receiver. While a tuned circuit may be used for this purpose in some cases, it Will be more desirable to use a sharply selective band lilter type oit circuit such as is :illustrated in Fig. 3. A. detector D, is also included in this branch, said detector being preferably of the type illustrated in Fig. hand having relays R2 and R3 controlled by its output circuit. The apparatus thus organized causes the `relays R2 and R3 to be energized during all the time that the 3,000 cycle generator G is connected to the radio transmitter T. Similarly, the receiving channclRE at the distant station provided with a branch including a selective circuit F', selective ot' the 100 cycle current which will be detected by the radio receiver it. Likewise, a detector D', is included in this branch and controls the relays It', and RQ. -Relays R2 and RQ, will therefore be energized during such times as the generator tlaplvilies 100 cycle current to the radio transmitter T.

The relays and connections used are suoli thatrelays R3 and It', close their contacts promptly on the cessation of the control current but permit a sullieient interval to elapse between the cessation oit the voice Waves and the opening ot the contacts to insure that all echoes lfrom the lines TL and TL have died out.

` A delay circuit FE, which may also function as a voice-pass lilter, is connected between. the point at which the detector branch l)L is applied to the transmitting channel TE and the point at which the contact of the relay B] is applied to the channel. This delay circuit may be, for example, a lowpass lilterfot the necessary number et sections such as illustrated in Fig. 2. A similar delay circuit FW is connected in the transmitting channel TW between the input bridge et the detector D, and the contact ot the relay ltl. The purpose ol these delay circuits Will presently appear. Tt is also desirable to provide voicepass lilter circuits such as FV and FE in the receiving channels RTW and. RE, respectively. 'These lilters may be ot the type shown in Fig. 2 or of the type of Fig. 3, desired. The purpose of these filters not to `introduce delay, but merely to select the detected voice currents from the correspondingradio receiver to the exclusion oit the detected control currents. y

Outgoing` transmission trom the subscribers station S, tor example, passes into the transmitting branch TE and through the delay circuit FE. While the `voice currents pass through the. delay circuit, a portion ot the voice energy enters the detector D1 (which may include suitable ampliliers) and upon being rectilied or partially rectified, it operates the relay R1. The relay R, disconnects the generator G from the radio transmitter T, and, at the same time, closes its front contact to complete the circuit ot the transmitting branch TE from the delay circuit FE to the radio transmitter T. The delay circuit FE should introduce sutiicient delay to permit this operation to take place before the beginning of the Wave leaves the delay circuit.

Normally, a steady control signal having f a frequency ot say 100 cycles is generatedv by the generator (l and impressed upon the radio transmitter T, thereby modulating the carrier. The modulated signal is radiated and picked up by the radio receiver lt which operates to detect the treopiency oit 100 cycles which is selected by the lilter l1" and impressed upon the detector 1)2. This detector rectil'ies or partially rectilies the received energy and thereby maintains the relays Rg and lln energized. Relay Rg maintains the receiving channel RE shortcircuited, so that the subscriber at the. station b" would .norn'ially hear nothing. Atteithis tone trequency is removed by the operation et the relay lh, as just described, relays RZ and ll, are deenergized. The former removes the short :ircuit from the receixfing channel RE to permit the trans mission to be received and transmitted over the line TL to the subscriber-s station S. The lvoice currents transmitted over the channel TE modulate the carrier ot the radio transmitter T and are received and detected by the radio receiver R and then selected by the voice lilter lPE 'tor transmission to the line TL. The delay circuit FE o'lf the transmitting channel TE delays this part ot the operation long enough tor the relay R1 to close its front contact, and for the relays Pfg and TV3 to be deenergized upon the cessation of the tone signal. The delay circuit FE may take care et the entire delay due to these iumrations. so that the ilter FE need only perform its selective ull bis

west end of the circuit.

this ease is also ineffective,

function. Therefore, thek filter FE; may` consist of only a few n, sections. The relay RQ, already referred to, as Ibeing released when the tone was removed, connects a short circuit across the transmittingvchannel TW to prevent the voice currents selected by the voice-pass filter-FE from passing 4back into the transmitting,channel TTV yas echoes. This short circuit is maintained un-pv til all echoes from the line TL have ceased. This short circuit also functions to short circuit the input of the detector DQ.

When voice transmission leaves' the radio transmitter T, some of it may pass directly to the radio receiverR associated with the Any harmful effect resulting` from this situation is prevented by having Vthe receiving' channel RW normally short cii'cuitedby the relay R2 which is held Aopeinted'by the 3,000 cycle current detected by the radiovieceiver R so long as the generator G at the distant end of the circuit is connected to the radio transmitter T. This tone continues to be ti'ansniitted'even while the subscriber at S is talking, due to the fact that the release of the relay R, short circiiited the input vside of the detector DQ, as already described, thus preventing the operation of thel relay R', either by echoes or by voice currents transmitted from the subscriber S. When the subscriber at S finishes talking, the relays restore the circuit to .the condition shown in the drawing.'

'to the control frequencies, and connecting these circuits in front of the detectors suoli as D2 and DB, very sharp discrimination is provided against static and'noise, thereby preventing false operation of the echo suppressing; relay equipment associated with the receiving branches. It will also be noted that in the normal condition of the,cii'cilit,

the carrier modulated in accordance with the tone from the generator (r is radiated from the transmitter T both tothe distant receiving` apparatus Rf and tothe local receiving apparatus R. The tonedetectedfby the latter apparatus is without effect, however, as itl can ynot be selected by the filter F. Similarly, of course, the'carrier modulated by a tone of 3,000 cycles is transmitted from the transmitting' apparatus T to the The detected tone i I ns the .filterv will nel; pass this frequency.

receiving apparatus R.

receiver R.

"adjust the potentiometer y i The tone frequency normallyradiated may be utilized to control the transmission level -of the receiving equipment as the transmissionv efliciency of the medium changes.A 'As is well known, the transmission efficiency of the ether varies or fades slowly, yparticularly during certain periods of the twentyfour-hour day, so that it is necessary to malte 'adjustments from time yto time to Vkeep thel will change accordingly, and the change in 1 amplitude of this tone vmay be usedvto automaticallyadj ust the transmission level.' For this purpose, a volume regulatory VR, schematically indicated in Fig. l, is connected to the output side of the selective circuit F to `control the efficiency of the radio receiver R, and, similarly, a volume regulator is yconnected to the output circuitof the selective circuit F' to control the efficiency ofthe regulators in their-operation may be understood from Fig. 5 whichy shows the details of the regulator VR. f

Referring to Fig. 5, a detector 'D3 is con'- nected to the output side of the filter F in `multiple with the detector D2 already ydescribed in connectionwith Fig. 1. A. voltlmetier relay is'connected to the output circuit of the detector D3. The armature of this relay is arranged to swing between two contacts which control relays 2l and 22,

respectively. These relays 2l and 22 are so arranged thatwhen either relay is energized, the armature circuit of the motor Mv Vis circuit [is closed,v so Vthat current flows through it in one direction, and when relay 22 energized, the current flows ythrough the field inthe oppositel direction. The motor M will therefore becaused to rotate in one direction oi' the other, depending,r upon which` of the two relays is energized. The inotorM is arranged to control yan adn justable potentiometer-Q3 by means of a suitable gearing or otherqmechanism 24. The

The detailed circuits of these closed. Then relay Qlis energized, the'iield llt) potentiometer 28 maybe arranged in the radio receiver R at any suitable point, as, for exam le, between a detector D and kamplifier ii, as shown.

When the tone frequency vselected by t-lie filter F is of normal amplitude, the rectified current appearing in the output circuit of the detector D3 will energize the winding of the volt'meter 20 to hold' its arniature in a neutral position with respect to the two con tacts. If the transmission medium becomes less efcient, the amplitude ofthe selected tone decreases and the armature of the relay motor Mito rotatein such direction as to to increase they bij gain of the amplifier A. 'lhe motor vM conH tinues to rotate in this direction until the tone frequency is brought back to normal, when the armature of the voltmcter relay 20 is again in a neutral position. It the transmission efliciency of the medium is increased, the voltmete'r relay 2O shifts its armature to say its right-hand contact, there` by energizing the relay 2l and causing the motor M to rotate in the opposite direction. rllhis causes an adjustment oi the potentiometer 23 Which reduces the gain of thc amplilier A until the `tone Frequency is again brought back to its normal level.

It will thus be seen that during the time that the tone frequency being radiated, the transmission elliciei'icy o'lE the receiving channels Will be automatically maii'itained ata normal level, regardless of' changes in the transmission medium. As soon, hoivever, as someone begins to tall; so that the tone is removed from the circuit, sonic provision must he made to prevent the volunie regulator from operating to change 'the setting of the potentiometer. 'llo accomplifish this result, a relay 25 is coin'iectcd in the output of the detector D3, and the contact o't' this relay is arranged so that when the relay is energized, battery Will be connected to the armature of the voltmeterrelay 2G. The relay 25 is suliiciently sensitive so that when `the amplitude `o't the tone is decreased, due to a change in the medium, the current passing through the Winding ot' the relay 25 Will be sul'licient to maintain it energized. When, however, the tone is completely interrupted, there will be no current flowing in the output side of the detector, and relay 25 Will therefore release its armature and prevent the armature ot the voltmeter relay from completing the motor circuit. Gonsequently, whenv anyone commences to talk, the adjusting apparatus Will remain in the condition in which it then happens to he until the talk ceases. As the 'fading which takes place in the type of system here under consideration is relatively slow, there vrill be no change in the volume level during the period of a brief conversation which will require any adjustment oi level.

lt will be obvious that the general principles herein disclosed may bc embodied in many other organizations 'widely dilliercnt from those illustrated Without departing from the spirit of the invention, as detined in the following claims.

What is claimed is:

l. ln a two-Way transmission system,r a

channel for transmitting in one directioir and another channel for transmitting in the opposite direction, an echo suppressor unit associated With one of said channels, Imeans for transmitting a controlling vfrequency over said channel when said channel is not used :tor signal transmission, said echo sup presser unit functioning as long as the controlling frequency is transmitted to maintain said channel disabled for signa-l trans mission, and means responsive to the application o'l signals to said channel to disconnect said controlling frequency whereby said echo suppressor unit renders the channel operative for signal transmission.

2. ln a two-Way transmission system, a channel for transmitting in one direction and another channel for transmitting in the opposite direction, an echo suppressor unit associated With each channel, means to nor-` mally transmit a controlling frequency over each channel during such times as it is not used for signal transmission, means whereby the echo suppressor unit associated with each channel maintains the channel disabled for signal transmission 'as long as the controlling frequency is applied, and means whereby when signaling currents are applied to either channel, the corresponding controlling lirequency is disconnected theretronnthercby permitting the corresponding echo suppressor unit to render the channel operative for signal transmission.

3. In a two-ivay transmission system, a channel for transmitting in one direct-ion and another channel for transmitting in the opposite direction, an echo suppressor unit responsive to voice currents associated With each channel near its transmitting end, means controlled by each unit for applying a controlling frequency to the channel during such times as voice currents are not applied, an echo suppressor unit associated with each channel near the receiving terminal thereof, each of said last mentioned echo suppressor units being responsive to the controlling frequency transmitted over the channel to disable the channel for voice transmission, and means whereby when voice currents are applied to a channel, the echo suppressor unit near its transmitting terminal disconnects the controlling trequency ltrom the channel, thereby causing the echo suppressor unit near the receiving terminal to render the channel operative for voice transmission.

fl. ln a two-Way transmission system, a channel for transmitting in one direction and another channel tor transmitting in the opposite direction, an echo suppressor unit responsive to voice currents associated with each channel near its transmitting end, means controlled by each unit for applying a controlling frequency to the channel during such times as voice currents are not applied, an echo suppressor unit associated With each channel near the receiving terminal thereof, each of said last mentioned echo suppressor units being responsive to the controlling frequency transmitted over the channel to disable the channel for voice transmission, means whereby when voice linfa currents are applied to a channel, the echo suppressor unit` near rits'transmitting ter-l minal disconnects the controlling, frequency from the'channel, thereby', causing the echo suppressor unit 'near the receiving terminal toL render the channel operative `for voice transmission, anda delay kcirc-uit associated with each vchannelito delay the transmission vof voice currents until theecho suppressor units have had time to perform their functions. Y -f L 5. In `a two-Way radiotelephone system,

a radio transmitter and a radio receiver at each terminal, transmitting connection .4

from the, radio transmitter and a receiving connection from the radio receiver jatfeach terminal to a corresponding terminaltelef.

receiving connection, saidlast mentioned suppressor unit responding to the tone ra diated by the distant transmitter to maintain the receiving connection inoperative, and means whereby when voice currents are applied to a transmitting connection, the echo suppressor unit associated therewith disconnects the controlling tone so that'the echo suppressor unit associated with the distant receiving connection renders said receiving connection operative for voice transmission. Y

6. In a two-way radio telephone system, a radio transmitter and a radio receiver at each terminal, a transmitting connection from the radio transmitter anda receiving connection from the radio receiver atL each terminal to a corresponding terminal telephone line, a voice-operated echo suppressor unit associated with each transmitting connection, means controlled-by said unit when` voice currents are not being transmitted forl applying a controlling tone to vthe radio transmitter, a sharply selective echo sup-v pressor unit associatedrwith each receiving connection, said last mentioned suppressor unit responding to the tone radiated by the distant transmitter tomaintainthe receiving connection inoperative, means whereby when voice currents are applied to a transmitting'connection, the echo suppressor unit associated therewithdisconnects the kcontrolling tone so that the echo suppressor unit associated with the distant receiving connection renders said receiving connection operative for voice transmission, and a delay circuit interposed between the echo suppressor of a transmitting connection and the corresponding radio transmitter to delay the transmission of voicer currents until the echo suppressor has had time-to operate. 7 In ,a5 two-Away radio telephone'system,

a `radio transmitter and a radioreizeiverat.

each terminal, a transmitting' connection `trom the radio transmitter and a receiving connection 'trom the radio receiver :at each' terminal to ,at corresponding terminal tele.w

phone line, fa voice-operated echo suppressor unitassociated with `each transmitting con*- nection, vmeanscontrolled by said e yunit when voice currents are. not beingtransmitted for applying a controlling tone, to the radio transmitter, a sharply selective echo-suppressoijunit associated with eachgreceiving connection, lsaid last mentioned suppressor vunitrespond-ing to the'tone radiated by the distanty .transmitter to` maintainl u the,v receiving connection "inoperative, means whereby when lvoice currents arelapplied, to altransmitting connectiomthe echo suppressor unit associated therewith .disconnects the controlf` ling tone so that the echosuppressory unit associated with the Vdistant .receivingconnection renders said receiving connection operative for voice transmission, and a volume regulator 'associated with each receiving con# nection and responsive to changes in thevolume of the received controllingtone to produce compensatingl changes in thel transmission efficiency ot the receiving channel, Wlierebyy the over-all transmission of the system is ,maintained substantiallyr constant.

8. In a two-way radio telephonesystenn a radio transmitterandaradio receiver at each terminal,y a transmitting connection from the radio transmitter and a receiving connection fromk the radio receiver at each terminal toafcorresponding terminal telephone line, a voice-operated echo suppressor unit associated` with each transmitting connection, meanscontrolled by said unit when voice currentsy are not being transmitted for .applying a controlling tone tothe radio transmitter, a sharply selective echo supv ypressor unit associatedy with each receiving connection, said last mentioned suppressor unit responding to the tone radiated by the distant transmitter to maintain the receiving connection inoperative, means whereby when voice currents areapplied to a transmitting connection, the echo suppressorunit yassociated therewith disconnects thecontrol ling tone so that the echo suppressor unit associated with the distant receiving connection renders said lreceiving connection opertive for voice transmission, a volume regu-y lator associated with each receiving connection and responsivet'o changes in the volumev Y of the received controlling tone to produce compensating changes in the. transmission efficiency of the receiving channel, whereby the over-ally transmission of the system is maintainedV substantially` constant, andv means to prevent said volume regulator from the channel is not being used for signal transmission, means associated with the receiving end 01": each channel and responsive to the controlling tone for maintaining the channel inoperative for signal transmission, and a volume regulator also associated 'With each channel near its receiving terminal,

said volume regulator being responsive to changes in the amplitude of the controlling tone to produce compensating adjustment in the transmission efficiency of the channel, 10. In a two-Way transmission system, a

channel for transmittingin one direction and a channel for transmitting in the oppolsite direction, means to apply a controlling tone to each channel during such times as the channel is not being used for signal transmission, means associated with the receiving end of each channel and responsive to the controlling tone for maintaining the channel inoperative for signal transmission, and a volume regulator also associatepl with each channel near its receiving terminal, said volume regulator being responsive to changes Vin the amplitude of the controlling tone to produce compensating adjustment in the transmission efficiency of the channel, said volume regulator being `unresponsive to complete interruption of the controlling tone. y

In testimony whereof, We have signed our names to this specica-tion this 22nd day of April, 1926.

,HARRY NYQUIST. SUMNER B. WRiGHT. 

